1. Field of the Invention
The present invention generally relates to battery module state detection method, and more particularly, to a detection method of measuring the internal resistance of the battery unit of the battery module during the regular discharging operation thereof as an index of the lifetime and quality of the battery module.
2. Description of the Prior Art
Generally, a real battery should have its own internal resistance comparing to an ideal battery. The internal resistance of the battery is determined by the internal factors such as the ion concentration in the electrolyte, as well as the area, density and material of the electrode plate. During the discharging process of the battery, the abovementioned internal factors will undergo changes in a forward direction and therefore result in the increasing internal resistance of the battery. When the battery is in the recharging process, the abovementioned internal factors will undergo changes in a reverse direction and result in a decreasing internal resistance of the battery. Therefore, there exists a positive correlation between the remaining energy and the internal resistance of the battery during the lifetime of a properly functioning battery. For example, if the internal resistance of the battery during the discharging processes increases to 1.24 times the internal resistance of the battery which is being fully recharged for the first time, it is indicated that the battery only has half of its full capacity left. The multiplier may vary according to the types of the batteries, but it should be almost the same value for batteries of the same type.
In practical application, however, with each time the battery is discharged or recharged, the ion concentration of the electrolyte and the area, density and material of the electrode plate will gradually degenerate and can't be recovered whether the process is in the forward or reverse direction. This situation reflects the fact that those inner factors can never be restored to their best state ever since the battery has been manufactured even if the battery is been recharged. As a result, the internal resistance will increase in proportion to the increasing times of the discharging and recharging process even if the battery is fully recharged in each recharging process. For example, when the internal resistance value of a recharged battery has increased to two times the internal resistance value of the battery after recharged for the first time, it is indicated that the battery should not be used anymore.
Since the internal resistance of the battery has characteristics of the variation mentioned hereinabove, the current remaining energy and residual lifetime of the battery can be predicted within its life cycle if there is a method for monitoring the internal resistance of the battery in real time. On the other hand, the internal resistance of the battery will increase rapidly near the end of the discharging cycle, which will induce a rapid rise of internal temperature.
Taking a battery module composed of a plurality of battery units into consideration. If a battery unit of the battery module comes to its end of the discharging cycle in advance while other battery units still work properly, it would be very difficult to point out such irregular operating condition from the operating state presented by the whole battery module. If the change of the internal resistance of respective battery units can be monitored in real time, then the abnormal battery unit can be detected in real time. In such a way, the position where electrical fire may occur in the battery module can be predicted and treated immediately to prevent the fire hazard.
To carry out the measuring of the internal resistance of a battery with the conventional method, an external high frequency alternating current signal has to be applied on the battery while the direct current has to be isolated by a capacitor. In addition, an AC meter is used to measure the voltage division of each battery and the total alternative voltage, which will then be used to calculate the internal resistance of the specific battery unit with reference to the internal resistance of the battery unit measured in advance. The shortcoming of this method is that the operating process and assembly of the equipment will be very complicated. For an electrical vehicle in service or other equipment in motion, it is nearly impossible to apply the aforementioned method to monitor the internal resistance of each of the battery units of the battery module in real time. What's more, dangerous situations may occur if the method is applied.
For example, the applied external alternating current will recharge the battery unit at a positive half-cycle and discharge the battery unit at a negative half-cycle. Therefore, if the detector device is connected on both side of the battery unit for a long period, it will cause great damage to the battery unit due to recharging and discharging the battery unit alternately with a high frequency, and meanwhile cause electric interference to the load equipment.